After multiple requests and complaints that the original was "too easy", I'm bringing in the new "Hardcore" version of this mod. Instead of smelting Rotten Flesh into Leather, you need to "purify" the Rotten Flesh first. Do this by putting Rotten Flesh, Sugar, and Bonemeal anywhere on the grid to make regular Flesh. Now only then you can make it into Leather by smelting it. Have fun!

Awesome! Thank you both for feedback. I've found similar mods, but they weren't updated, and the only updated one required 9 rotten flesh in a crafting grid to make one leather so I just decided to make my own.